The present invention relates generally to diagnostic imaging and, more particularly, to a method and apparatus of correcting bad cell data acquired in an imaging detector.
Typically, in computed tomography (CT) imaging systems, an x-ray source emits a fan-shaped beam toward a subject or object, such as a patient or a piece of luggage. Hereinafter, the terms “subject” and “object” shall include anything capable of being imaged. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing system for analysis which ultimately produces an image.
Generally, the x-ray source and the detector array are rotated about the gantry within an imaging plane and around the subject. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal point. X-ray detectors typically include a collimator for collimating x-ray beams received at the detector, a scintillator for converting x-rays to light energy adjacent the collimator, and photodiodes for receiving the light energy from the adjacent scintillator and producing electrical signals therefrom.
Typically, each scintillator of a scintillator array converts x-rays to light energy. Each scintillator discharges light energy to for instance a backlit photodiode adjacent thereto. Each photodiode detects the light energy and outputs a corresponding electrical signal. The outputs of the photodiodes are then transmitted to the data processing system wherein a digital signal is generated, stored, and used for image reconstruction. Such devices may be used in conventional CT, x-ray, mammography, and tomosynthesis applications.
The backlit photodiodes are attached and electrically connected to a multi-layer substrate that carries the electrical signals from the back side of the photodiode to the data processing system through a flexible electrical circuit. Accordingly, for each pixel within a CT detector, the scintillator is optically coupled to the photodiode, and an electrical contact is typically made between the photodiode and the substrate, and between the substrate and the flexible electrical circuit. When a detector pixel develops an open or short condition, it is referred to as a “bad cell” or “bad pixel” and produces an insufficiently measured digital signal or no signal at all. A short condition can occur between a pixel and its neighbor pixel or between a pixel and ground. An open condition is primarily due to a completely disconnected pixel and tends to leak current into the neighboring pixels.
Typically, when imaging data is acquired without bad pixels, features within an image appear distinct from other features within the image. However, bad pixel data manifests itself as streaks or other image artifacts. Several algorithms are commonly known and applied to correct bad cells, such as, for instance, linearly interpolating values for a missing pixel by using surrounding neighbor pixels. However, when a bad cell or pixel occurs in an image near a sharp edge, such as, for instance, at a boundary between a high density material and a low density material, linear or higher order interpolation can result in over- or under-estimating the value of the pixel, thus increasing the propensity to cause streaks or image artifacts. Furthermore, bad cells or pixels may occur in blocks of, for instance, 3×1 or 3×3 cells and the like, thus exacerbating the problem.
Additionally, to improve resolution and performance, detector geometries other than conventional orthogonal grids are under consideration separate from, or in conjunction with, implementation of a wobbled focal spot. For instance, a diagonal detector geometry may improve resolution of a CT system. However, because of the increased processing complexity of building a diagonal cut detector, a diagonal detector may result in an increased number of bad cells. Additionally, because of the diagonal cut geometry, conventional interpolating algorithms are not sufficient to completely eliminate streaks and artifacts from images.
Therefore, it would be desirable to design a system and method to improve correction of data from bad cells in a CT detector for orthogonal pixel patterns and diagonal cut pixel patterns.